The way I understand it is that If the REAL plane has a 36 foot wingspan a 35% model would have a 12'6" wingspan.
12'6" is 35% of 36' (so forth and so on)
I also have been told that it's not just the wingspan. All the dimensions of the plane are sposed to be close to the particular percentage of the real one in order to be a called a percentage plane of whatever particular percentage is being stated.
Say that 3 times fast!
(did ya get that??)
You'll have to wait till some others chime in here to confirm what I just told ya though.